This invention is generally related to laser shock peening and, more particularly, is related to a method and system for controlling dual laser shock peening of an article.
Gas turbine engines and, in particular, aircraft gas turbine engines rotors operate at high rotational speeds that produce high tensile and vibratory stress fields within the blade and make the fan blades susceptible to foreign object damage (FOD). Vibrations may also be caused by vane wakes and inlet pressure distortions as well as other aerodynamic phenomena. This FOD causes nicks and tears and hence stress concentrations in leading and trailing edges of fan blade airfoils. These nicks and tears become the source of high stress concentrations or stress risers and severely limit the life of these blades due to High Cycle Fatigue (HCF) from vibratory stresses.
Thus, it is highly desirable to design and construct longer lasting fan and compressor blades, as well as other hard metallic parts, that are better able to resist both low and high cycle fatigue and that can arrest cracks better than present day parts. The below referenced U.S. Patent Applications or U.S. Patents are directed towards this end: U.S. patent application Ser. Nos. 08/993,194, now U.S. Pat. No. 5,932,120, entitled xe2x80x9cLaser Shock Peening Using Low Energy Laserxe2x80x9d; Ser. No. 08/362,362, xe2x80x9cOn The Fly Laser Shock Peeningxe2x80x9d, filed Dec. 22, 1994, now U.S. Pat. No. 6,215,097; and U.S. Pat. No.: 5,591,009, entitled xe2x80x9cLaser Shock Peened Gas Turbine Enginer Fan Blade Edgesxe2x80x9d; U.S. Pat. No. 5,569,018, entitled xe2x80x9cTechnique To Prevent Or Divert Cracksxe2x80x9d; U.S. Pat. No. 5,531,570, entitled xe2x80x9cDistortion Control For Laser Shock Peened Gas Turbine Engine Compressor Blade Edgesxe2x80x9d; U.S. Pat. No. 5,492,447, entitled xe2x80x9cLaser Shock Peened Rotor Components For Turbomachineryxe2x80x9d; U.S. Pat. No. 5,674,329, entitled xe2x80x9cAdhesive Tape Covered Laser Shock Peeningxe2x80x9d; and U.S. Pat. No. 5,674,328, entitled xe2x80x9cDry Tape Covered Laser Shock Peeningxe2x80x9d, all of which are assigned to the present Assignee. They teach to provide an airfoil of a fan blade with a continuous or volumetric region of deep compressive residual stresses imparted by laser shock peening over at least an inwardly extending portion of laser shock peened surfaces of an article, such as the fan blade. These regions are formed by multiple overlapping protrusions of compressive residual stresses imparted by laser shock peening that extend inward from overlapping laser shock peened circles or spots.
The deep compressive residual stresses imparted by laser shock peening of the present invention is not to be confused with a surface layer zone of a work piece that contains locally bounded compressive residual stresses that are induced by a hardening operation using a laser beam to locally heat and thereby harden the work piece such as that which is disclosed in U.S. Pat. No. 5,235,838, entitled xe2x80x9cMethod and apparatus for truing or straightening out of true work piecesxe2x80x9d. The prior art teaches the use of multiple radiation pulses from high powered pulsed lasers and large laser spot diameters of about 1 cm to produce shock waves on the surface of a work piece similar like the above referenced Patent Applications and U.S. Pat. No. 3,850,698, entitled xe2x80x9cAltering Material Propertiesxe2x80x9d; U.S. Pat. No. 4,401,477, entitled xe2x80x9cLaser shock processingxe2x80x9d; and U.S. Pat. No. 5,131,957, entitled xe2x80x9cMaterial Propertiesxe2x80x9d. Laser shock peening as understood in the art and as used herein, means utilizing a laser beam from a laser beam source to produce a continuous region of strong compressive residual stresses in a continuous region on a portion of a surface. The region is volumetric and produced by the coalescence of individual protrusions extending inward from overlapping laser shock peened circles or spots. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled xe2x80x9cLaser Peening System and Methodxe2x80x9d. Manufacturing costs of the laser shock peening process is a great area of concern because startup and operational costs can be very expensive. The xe2x80x9con the flyxe2x80x9d laser shock peening process disclosed in U.S. Pat. No. 6,215,097, above is designed to provide cost saving methods for laser shock peening as is the present invention. Prior art teaches to use large laser spots, on the order of 1 cm and greater in diameter, and high powered lasers. Manufacturers are constantly seeking methods to reduce the time, cost, and complexity of such processes. A laser shock peening method that uses a low power laser beam, on the order of 3-10 joules, with a preferred range of 3-7 Joules and laser beam spots having a diameter of about 1 mm is disclosed in co-pending U.S. Pat. No. 5,932,120, entitled xe2x80x9cLaser Shock Peening Using Low Energy Laserxe2x80x9d and this method is directed to reducing time, cost, and complexity of laser shock peening. There is an ever present desire to design techniques that result in such reductions and to this end the present invention is directed.
As suggested above, known prior art laser peening techniques have been solely concerned with the use of a single laser beam that struck the surface to be peened at a designated position, at a designated angle. Recent advances in laser shock peening technology may require that the article be simultaneously struck on mutually opposite surfaces so that the respective shock waves created by the two impinging laser beams meet at the center of the opposite surfaces. See U.S. Pat. No. 6,005,219, issued to the same Assignee of the present invention and herein incorporated by reference. Manufacturing use of this dual laser technique imposes a need for developing programmable tools, such as numerical control (NC) tools, that allow for accurately, reliably and inexpensively controlling the dual laser peening process.
Thus, it is desirable to be able to provide an automated process for developing commands for controlling a dual laser shock peening device using presently available NC part-positioning technology. It is further desirable to be able to accurately and quickly position the article relative to the dual laser beams so that each laser beam simultaneously impinges on a respective spot situated on either of the mutually opposite surfaces. Since the respective spots which are simultaneously struck by the two laser beams lie opposite one another on mutually opposite surfaces of the article, it is also desirable to be able to determine the precise location of the spots at which the beams must strike the article to achieve a desired surface coverage and strong compressive residual stresses. As suggested above, it would also be desirable to be able to determine the command strategy, e.g., NC commands, to be programmed into the article-positioning device to three-dimensionally align the article to spatial locations that achieve the appropriate coverage.
Generally speaking, the present invention fulfills the foregoing needs by providing a method for dual laser shock peening an article. The method allows for defining a spot pattern comprising a plurality of spots on a first surface of the article to be peened. The method further allows for defining a spot pattern comprising a plurality of spots on a second surface of the article to be peened. The first and second surfaces comprise mutually opposite surfaces relative to one another. Each one of the respective spots on the second surface is arranged to correspond to a respective spot on the first surface and comprising a plurality of matched pair of spots. A generating step allows for generating dual laser beams being respectively aligned to simultaneously impinge on each respective matched pair of spots.
In another aspect of the invention, the foregoing needs are further fulfilled by providing a system for dual laser shock peening an article. The system comprises a spot pattern generator for defining a spot pattern comprising a plurality of spots on a first surface of the article to be peened. The pattern generator further defines a spot pattern comprising a plurality of spots on a second surface of the article to be peened. The first and second surfaces comprise mutually opposite surfaces relative to one another. Further, each one of the respective spots on the second surface is arranged to correspond to a respective spot on the first surface and comprises a plurality of matched pair of spots. A laser unit allows for generating dual laser beams that are respectively aligned to simultaneously impinge on each respective matched pair of spots.